1. Field of the Invention
The present invention relates to a method of manufacturing a metallic oxide, and more particularly, to a method of manufacturing a metallic oxide which can be utilized as a superconductive material exhibiting superconductivity at a specific temperature or below and applied to, for example, superconductive coils for power storage, magnetic shields, various electronic devices, such as Josephson devices, and superconductive magnets for use in medical apparatus and high-precision measuring apparatus, such as MRI and SQUID, as well as a metallic oxide manufactured by such a method.
2. Description of the Related Art
The superconductive materials which are currently available on the market or whose practical use has been examined are Nb, NbTi, Nb3Sn and Nb3Al. These conventional superconductive materials, however, have to be cooled to very low temperatures by liquid helium to make them superconductive. Further, these superconductive materials do not show superconductivity under a magnetic field of about 20 tesla or above. Such a characteristic is inherent in these materials, and thus a superconductive material having a higher critical temperature than the above-described conventional superconductive materials is required to allow a magnetic field of 20 tesla or above to be generated. However, the existence of such a material is not known in the art, and the use of a superconductive material under a magnetic field of 20 tesla or above has hence been basically impossible.
In recent years, various perovskite compounds containing Cu have been discovered. Among them are materials which exhibit superconductivity at a temperature higher than the boiling point of liquid nitrogen, such as Y--Ba--Cu--O (hereinafter referred to as a Y type), Bi--Sr--Ca--Cu--O (hereinafter referred to as a Bi type), or Tl--Sr--Ca--Cu--O (hereinafter referred to as a Tl type). These oxide superconductive materials have a higher critical temperature and a larger critical magnetic field than the conventional superconductive materials, and, due to their potential application in fields to which conventional superconductive materials cannot be applied, have been researched. However, Bi or Tl type superconductive materials are toxic, and hence cannot be used except under special conditions, from the viewpoint of safety. Furthermore, these superconductive materials are not chemically stable under conditions where water vapor and carbon dioxide co-exist. Also, they have a complicated crystal structure in which crystal phases having a plurality of different critical temperatures can co-exist, and provision of a Bi or Tl type superconductive material having a crystal structure of a single crystal phase is difficult. Further, Bi or Tl type superconductive materials have a very small lower critical magnetic field in which the superconductive material can be both superconducting and normally conducting. Y type superconductive materials are characterized in that the content of oxygen present near the surface of the material readily changes. Accordingly, when such Y type superconductive materials are employed as wire materials used for electronic devices or magnets, an interface between the superconductive material and a different type of material or an interface between the crystal grains of the superconductive material is not stable, making the characteristics of the product unstable. Therefore, despite the advantages of high reactivity with water contents and a larger lower critical magnetic field than Bi type materials, Y type superconductive materials cannot exhibit an excellent practical performance.
Among the superconductive materials containing halogen, for example, a Y type superconductive material with fluorine introduced therein is disclosed in Japanese Patent Laid-Open No. sho 63-274657. Japanese Patent Laid-Open No. hei 01-100001 discloses a ABCOD material (A represents an element forming IIA group of the periodic table, B represents an element forming IIIA group of the periodic table, C represents an element forming IB group of the periodic table, and D represents fluorine). Among the superconductive material manufacturing methods, a method of introducing fluorine by ion injection has been proposed in Japanese Patent Publication No. hei 04-40284, and a treating method by ClF3 gas under a low pressure is disclosed in Japanese Patent Laid-Open No. hei 4-124003.
Although Y, Bi and Tl type superconductive materials having a critical temperature higher than the boiling point of liquid nitrogen are attractive materials, the practical use of such superconductive materials is limited due to safety problems and a deterioration in the reproducibility of the product characteristics caused by chemical instability. Japanese patent Laid-Open No. sho 63-274657 discloses a Y type superconductive mateirial into which fluorine is introduced at a concentration of 1/100 to 200% of the oxygen vacancy in the material. However, as the concentration of oxygen vacancy is unclear, the amount of fluorine in the superconductive material is unclear. Although the above-described prior art describes that 3.times.10.sup.21 cm.sup.-3 of fluorine are introduced into the Y type material by ion injection, it is inferred from this disclosure that the introduced fluorine is present in the material in the vicinity of the surface thereof. Also, a film of, for example, SiN is formed on the surface of that superconductive material. As a whole, the superconductive material disclosed in the above-described prior art is considered a material into which fluorine is introduced non-uniformly. Further, although the above-described prior art describes the instability of the portion which is 200 .ANG. from the material surface, it does not describe the chemical stability of the material as a whole. The material disclosed in Japanese Patent Laid-Open No. hei 01-100001 has a composition of Y1Ba2Cu3F0.2Oy and a critical temperature of 125 K. It is, however, difficult to manufacture, with good reproducibility, a Y type superconductive material having such a high critical temperature. The thus-manufactured Y type superconductive material has a problem involving reliability. In addition, there is no description in the above-described prior art about the chemical stability of the material.
Among the methods of manufacturing a superconductive material containing halogen, the halogenation method by ClF3 gas disclosed in Japanese Patent Laid-Open No. hei 4-124003 requires not only a normally employed electric furnace but also special parts and special devices, such as a container made of Ni, an evaporating device, flow control of gas and treatment of exhaust gas, and hence cannot be readily applied to the manufacture of a large product. Further, this prior art teaches that even when the amount of oxygen in Y--Ba--Cu--O material is low, the on-set superconductive transition temperature can be improved. However, an increase in the weight per gram of the sample obtained in the above-described method by halogenation is from 0.01 to 0.05 grams. It is therefore inferred that chemical stability cannot be improved even through the critical temperature may be improved due to a high halogen substitution rate for oxygen. Japanese Patent Publication No. hei 04-40284 teaches the method of introducing fluorine by ion injection, and requires coating of SiN or AlN on the surface of the superconductive material. In this method, even if fluorine can be introduced into the vicinity of the material surface, it cannot be introduced uniformly over the entire material having a certain thickness. This method, which employs, as a material substance, a metal halide, is simple. However, it is difficult to accurately control reaction of the oxide with a halide in this method. Particularly, substitution of halide for special oxygen atoms can barely be controlled.